(at around 35 mins) When John Russell first examines the notebook in the attic room, the sticker on the cover says: "C.S.B., January 1909". When he brings Claire Norman to the attic room (at around 38 mins), she picks up the notebook and reads "C.S.B., January FOURTH 1909" off the cover.
(at around 50 mins) When the séance resumes for the second time, a camera shot angle shows the metal cone in the center of the table surrounded by the large pieces of paper the medium wrote/scribbled on earlier, however when the camera pans back a moment later when the spirit enters the room from the staircase, the papers are neatly stacked and the cone is off center.
(at around 43 mins) John's daughter's ball is red and gray with a white stripe up to the point where it bounces down the stairs the first time. When John takes it to the bridge and when it bounces down the stairs the second time, it is all red with a white stripe.
The level of the flames changes inconsistently several times during the end sequence.
(at around 1 min) In the beginning of the movie when John Russell is shown walking across the road to the phone booth, the road is packed solid with snow. When it shows him reaching the phone booth, in the background you can see that cars have cleared a path with their tires.
INCORRECTLY REGARDED AS A GOOF: (at around 1 min) There are no telephone poles or wires going to the phone booth in the opening scene. The majority of telephone booths were connected by underground wiring.
(at around 10 mins) When Claire is showing John the piano room the camera moves backwards and it reveals the shadow of the camera. The camera also touches the piece of cloth covering the piano and you can see it moving.
(at around 1h 07 mins) John and Claire are driving to Mrs. Gray's house and the driver of the car is clearly not John but a younger man with short, dark hair.
When John and Claire are in the library looking for information on one of the previous owners of the house, a librarian assists them by taking a microfilm of old newspapers from 1909 out of a drawer. He tells them he'll set it up on the viewer for them, but as they walk over to the viewer (still in the same shot) it is miraculously already loaded and on the viewscreen.
When John and Claire are in the library looking for information on one of the previous owners of the house, they are reading the article, it shows the month as Februry.
When John enters Joseph's room for the first time, none of the extensive spider webs inside move at all. The room is out of the way but even with one panel of the stained glass window gone, the webs would still move a little from the wind.
After John Russell has finished listening to the reel to reel recording from the séance, the take-up spool continues to spin AFTER the feeder spool has emptied. Most, if not all machines like this would AUTOMATICALLY stop with no tape being fed through.
When John goes to the third floor following the sound of running water, he ends up at an archway overlooking the attic. In his view, he sees two doors beyond the arch, but after going through, the second door is now replaced by another archway.
After the glass shatters in the attic garret window, John returns to the attic to investigate. Standing at the third floor archway, he can be seen with his back to a third floor railing, but looking from the archway out, John's back is to a solid wall with a window in it.
(at around 34 mins) After John Russell discovers the secret stairs he gets to a room with an open door. When he looks into the room, the camera moves back and reveals a shadow of a human head disappearing on the left side of the screen. Either this is fully intentional by the filmmaker or it's a shadow of a crew member.
(at around 51 mins) During the second part of the séance when the metal flute object is in the center of the table, the camera's point of view descends the staircase and walks into the room. The shadow of the camera operator is visible on the floor to the left, just after entering the room.
(at around 36 mins) The next scene after John discovers the small child's room and the music box, Claire is listening to music on the reel to reel tape player. There is a large mirror and several small mirrors built into the mantle above the fireplace behind her. As the camera dollies right towards John, you see the arm of a grip pushing the camera in the small right mirror below the single large mirror.
The locations in the movie are stated as Seattle and Spokane but the movie ignores the 300 mile distance between these cities. For example, the ambulance leaving Carmichael's residence in Spokane drives past locations in Seattle en route to a hospital.
After Claire comes running down the stairs screaming she yells to John but mistakenly calls him George for George C. Scott. This has been removed from some versions.